1. Field of the Invention
The present invention relates to a gas sensor having ceramic members, such as a holder, a sleeve and a separator, and used as, for example, an air/fuel ratio sensor.
2. Description of the Related Art
Conventionally, an air/fuel ratio sensor, for example, includes the following ceramic members: a holder through which a platelike gas detection element (hereinafter, referred to as a detection element) is inserted and which is engaged with a forward portion of the gas detection element; a sleeve through which the detection element is inserted and which is engaged with a central portion of the detection element; and a separator which accommodates a rear end portion of the detection element and into which connection terminals made of metal are inserted (refer to Patent Documents 1 and 2).
Among the above-mentioned ceramic members, as shown in FIG. 20, the holder and the sleeve have, for example, an element insertion hole P2 through which a detection element P1 (having a substantially rectangular cross section) is inserted. FIG. 20 is a view of the sleeve as viewed from the direction of the axial line of the detection element P1. The element insertion hole P2 has a substantially quadrate shape surrounded by four sides; however, corners P3 of the element insertion hole P2 are not right-angled (at an angle of 90 degrees), but are radiused; i.e., smoothly curved. This is for the following reason: as a result of employing right-angled corners P3, stress is apt to be imposed on the apexes of the corners P3, potentially generating cracks in the ceramic member.    [Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2006-308328    [Patent Document 2] Japanese Patent Application Laid-Open (kokai) No. 2007-3216
3. Problems to be Solved by the Invention
However, when the edges P3 of the element insertion hole P2 are radiused, as shown in FIG. 20, the dimensions (vertical dimension T1 and horizontal dimension T2) of the element insertion hole P2 must be greater than the dimensions (specifically, thickness t1 and width t2) of the detection element P1 to be inserted into the element insertion hole P2.
This is for the following reason: when the platelike detection element P1 is to be inserted into the element insertion hole P2, the radiused surfaces of corners P3 of the element insertion hole P2 must be located radially outward of the apexes of edges P4 of the platelike detection element P1.
As a result, a clearance between the side surfaces of the detection element P1 and the inner peripheral surfaces of the element insertion hole P2 increases, thereby raising a problem in that the detection element P1 is apt to have play within the element insertion hole P2, with a resultant deterioration in positioning accuracy.
Although unillustrated, in the case of the separator, when the connection terminals are to be disposed through insertion into terminal insertion holes, similar to the above-mentioned case, since the edges of the terminal insertion holes are radiused, a problem arises in that the connection terminals are apt to have play, with a resultant deterioration in positioning accuracy.